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What are Instrument Transformers?

By Paul Scott
Updated May 17, 2024
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Instrument transformers proportionally step down or reduce large voltage and current values so that they may be safely used on sensitive instruments and components. These instruments and components include ammeters, voltmeters, and overcurrent protection relays. Instrument transformers fall into two distinct categories: potential and current transformers. A potential transformer (PT) is used to supply a low voltage to voltmeters, and a current transformer (CT) supplies manageable current vales for ammeters and overcurrent relays. PTs may be co-located with the high tension supply in local starter panels or remotely in instrument control panels while CTs are generally co-located.

High tension power supply installations may carry voltages as high 30,000 volts with common distribution voltages in excess of 13,000 volts. Three phase equipment such as electric motors in industrial installations commonly run on 380 or 500 volts and often draw several hundred amps during operation. The average voltmeter has a maximum rated voltage of 110 to 150 volts and could never directly measure these voltages. To allow for the use of these components, precision instrument transformers proportionally reduce the high voltages and currents to safe values. Instrument transformers also allow the installation of all required instrumentation in control rooms and monitoring facilities with only low voltage cabling present.

Installation transformers fall into one of two separate categories. The first is the potential transformer which supplies low value voltages to voltmeters. The PT is typically equipped with four terminals; two for the incoming high voltage and two for the reduced instrument voltage. The PT's incoming or primary leads are connected in parallel across the high tension supply. This means that it may be located remotely in control rooms although it can often be mounted in high tension starter panels where local voltage indication is required.

The second type is the current transformer. This instrument transformer variant measures current and is used to drive ammeters and overcurrent protection relays. The typical CT differs from its voltage-reliant sibling in that it is connected in series with the supply and typically utilizes an induced primary feed and not a direct connection. To achieve this, a CT is produced with a hole in its center through which a high tension cable runs. The current passing through the cable induces an electromagnetic field in the CT's primary winding which is then stepped down by its secondary winding and fed to the instruments via the CT's two output terminals.

Instrument transformers are available in a large range of designs and ratings and may be custom built to suit all user requirements. Common PTs and CTs are fairly small components, seldom measuring more than a couple of inches square. Large, oil bath instrument transformers are, however, commonly used to facilitate low voltage measurement in primary distribution grid applications.

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